This application claims the priority of Korean Patent Application No. 2002-21644, filed on Apr. 19, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an adaptive loop gain control circuit for a voltage-controlled oscillator (VCO), and more particularly, to an adaptive loop gain control circuit which compensates for a loop gain according to changes in an operating voltage and changes in temperature of a voltage-controlled oscillator (VCO) used in a phase locked loop (PLL) system.
2. Description of the Related Art
A phase locked loop (PLL) circuit synchronizes the phase of a received signal, where an oscillator or a periodic signal generator is controlled to operate at in-phase, synchronizes with respect to a reference signal. The PLL circuit has been used in various applications, including synchronous demodulation of a digital modulated wave, coherent carrier tracking, extension of a threshold, synchronization of a bit, and synchronization of a symbol.
FIG. 1 is a block diagram of a general phase locked loop (PLL) system.
Referring to FIG. 1, the PLL system includes a phase detector (PD) 110, a low pass filter (LPF) 120, and a voltage-controlled oscillator (VCO) 130 connected such that an output frequency signal of the voltage-controlled oscillator 130 is sent to the phase detector 110. The voltage-controlled oscillator 130 includes a voltage-to-current converter 131 and a ring oscillator 132. The voltage-to-current converter 131 converts an input control voltage VCON developed by an impedance of the low pass filter 120 into a control current ICON used to control an oscillation frequency corresponding to the input control voltage VCON and supplies the control current ICON to the ring oscillator 132. The ring oscillator 132 includes a plurality of current switches S1 through SN that are connected in a ring-like arrangement where an input is connected to an output having an opposite polarity to that of the input to convert the control current ICON into the oscillation frequency. An oscillation frequency fVCO proportional to the control current ICON in the ring oscillator 132 having the plurality of current switches S1 through SN is calculated as follows,                               f          VCO                =                              N            ·                          I              CON                                            C            ·                          V              O                                                          (        1        )            
where C denotes a capacitive load of the current switches, and VO denotes the strength of the oscillation signal.
A frequency gain KVCO of the voltage-controlled oscillator 130, which converts an input control voltage signal into an output frequency signal, can be defined as follows.                               K          VCO                =                              ∂                          f              VCO                                            ∂                          V              CON                                                          (        2        )            
Therefore, the frequency gain KVCO of the voltage-controlled oscillator 130 with respect to the control current ICON can be expressed as follows.                               K          VCO                =                              ∂                          I              CON                                            C            ·                          V              O                                                          (        3        )            
In addition, the gain A of the PLL system can be expressed as follows,                     A        =                                            K              φ                        ·                          K              VCO                        ·                          Z              ⁡                              (                S                )                                              S                                    (        4        )            
where Kxcfx86 denotes a phase difference gain of the phase detector 110, Z(S) denotes an impedance of the low pass filter 120, and S denotes a frequency domain.
According to equation 4, the gain A of the PLL system is primarily proportional to the frequency gain KVCO of the voltage-controlled oscillator 130.
However, if there is a change in the operating environment such as a decrease in operating voltage or an increase in operating temperature, the frequency gain KVCO of the voltage-controlled oscillator 130 decreases. Therefore, the gain A of the PLL system decreases in proportion to the decrease in the frequency gain KVCO of the voltage-controlled oscillator 130. Thus, the input control voltage VCON has to be increased in order to maintain a predetermined oscillation frequency. The voltage-controlled oscillator 130 of the PLL system is sensitive to changes in the gain A of the PLL system under certain operating environments.
The present invention provides an adaptive loop gain control circuit for a voltage-controlled oscillator (VCO), which is capable of maximizing a linear region of an input control voltage and compensating for a gain of the PLL system in response to changes in the operating environment, in order to ensure wide operating stability of a phase locked loop (PLL) system.
According to an aspect of the present invention, there is provided a adaptive loop gain control circuit comprising a detected voltage generating unit which generates a detected voltage signal in response to changes in the operating voltage and operating temperature; and a control circuit unit which outputs an oscillation control current signal according to the detected voltage signal and an input control voltage signal.
It is preferable that the detected voltage generating unit comprises a filter which eliminates noise of the detected voltage signal. The filter is an RC filter comprising a resistor and a capacitor.
The detected voltage generating unit comprises a first MOS transistor which has a gate and a drain that are connected to a first supply voltage and a source that outputs the detected voltage; and a second MOS transistor which has a gate and a drain that are connected to a second supply voltage and a source that is connected the source of the first MOS transistor.
It is preferable that the control circuit unit comprises a first operational amplifier where the input control voltage signal is inputted to a non-inverting terminal and an inverting terminal is connected to an output terminal; a second operation amplifier where the detected voltage signal is input to a non-inverting terminal, an inverting terminal is connected to the output terminal of the first operation amplifier, the second operation amplifier amplifying a difference between output voltages of the detected voltage and the first operational amplifier and outputting the current control voltage signal; and a third MOS transistor which has a gate connected to the output terminal of the second operational amplifier and a source connected to the first supply voltage and outputs the control current signal to the first supply voltage. Here, it is preferable that the adaptive loop gain control circuit further comprises a filter comprising a capacitor and a resistor connected each other in parallel between the inverting terminal and the output terminal of the second operational amplifier.